Process of heat treating iron sulphide material



5- W. YATES Sept, 20, 1932.

' PROCESS OF- HEAT TREATING IRON SULPHIDE MATERIAL Filed April 4, 1928 94.40% 41% I wuv-dd au A TTORNE Y.

Patented Sept. 20, 1932 UNITED STATES PATENT. OFFICE f SAMPSON W; YATES, OF CINCINNATI, OHIO, ASSIGNOR TO THE RICHARDSON COMPANY,

F LOCKLAND, OHIO, A CORPORATION OF OHIO PROCESS or HEAT TREATING more SOLPHIDE MATERIAL Application filed April 4, 1928. Serial N0. 267,303.

My invention relates to the heat treating of iron sulphide materials and particularly ferrous sulphide, so that sound castings can be made of this material; and my process is particularly valuable in connection with electrolytic processes utilizing anodes of ferrous sulphide material. Such an electrolytic process is described in the co-pending application of John R. Cain, Serial No. 238,232, filed December 6, 1927, entitled Processof plating soft ironi 7 In accordance with this process soft, malleable and ductile electrodeposits of iron are produced through the use of soluble anodes in a hot neutral ferrous electrolyte such as ferrous chloride. The desired characteristics in the iron deposit seem to be produced at controlled pH values in the electrolyte (in combination with the other factors). The maintaming of an electrolyte under conditions of continuoususage in: a ferrous condition and at a desired pH value, i. e. the conditioning of the electrolyte, is a major problem with which the said invention of John R. Cain is concerned; and he has found that anodes of ferrous sulphide material in hisprocess act not only as a source of iron, but also as conditioners to maintain a ferrous condition of the electrolyte and control its hydrogen ion concentration. i

It will be seen that for use as anodes in any considerable size, sound castings of the ferrous sulphide materials are requisite. Cracked castings conduct the current poorly and disintegrate unevenly, whereas broken castings are either unusable at all or, if usable, require a plurality of contacts and much expense in handling In another application filed herewith andentitled Processes of producing ferrous sulphide material, in the names of John R. Cain and myself, there is described a method of producing a ferrous sulphide material in a cupola or shaft furnace' One procedure outlined in this application is to crush hematite iron ore, cokeand iron pyrite'or ferric sulphide of iron and to briquette mixed portions of the material containing the requisite percentages of iron ore, reaction coke and pyrite. The briquettes are treated in a furnace which may be. of the ordinary cupola type,

and under the influence of heat the'hematite iron-ore is reduced and the iron combines with an atom of sulphur released by the heat from v the ferric sulphide. Molten ferrous sulphide V collects in the bottom of the cupola and may be tapped off. 'My;present invention ,is applicable to the ferrous sulphide material produced by this process or by any other process I of producing it in a molten condition; and obviously it will'be equally applicable to the casting of ferrous sulphide materials which have been remelted.

Heretofore, while therehas been some variation in the-iron sulphide product'manufactured by the process above referred to andby other processesysuch variations depending somewhat upon the nature of the charge and the conditions in the furnace, I have found that the'ferrous sulphide materials have a tensile strength generally in the neighborhood of and averaging 450 pounds per square inch.

Further, these ferrous sulphide materials have essentially'a smooth coolingcurve and also a smooth expansion curve. The expansion and contraction thereof with heat is great, and when the material is taken from the cupoIa or shaft furnace in a molten condition and poured from a ladle into moulds suitable for the production of anodes, as cooling proceeds there is such acontraction of the outer; or cooler layers of the casting while i 'thecentral part is still quite hot as-to cause cracks and fissures to form' therein'when the two portions, after solidification, approach room temperature. Frequently entire cor-. ners will break off the casting, and thecasting itself may split into two or more pieces.

This action is acceleratedby the rapidity of cooling; but have not been able to make sound castings of ferrous sulphide excepting by the heat treatment which I shall describe in these specifications.

increasing the tensile strength. -.I have developed a heat treatment which will increase the tensile strength practically fifty per cent, and produce a sound casting, free from cracks.

In the drawing h Figure 1 is a reproduction of a micro-photograph at 100 diameters of a representative sample of ferrous sulphide material from an unheat-treated casting. v Figure 2 is a representation of a photograph at the same magnification of a heat treated sample thereof, taken from the same lot and having the same composition.

It should be pointed outthat the applicability of my process is not limited to pure ferrous sulphide (FeS) The iron sulphide material that is made in thecupola orshaft furnace by the process referred to above may containvarious percentages of iron and'of sulphur, depending upon the charge and the furnace conditions. It is possible that at times a material is obtained which has the approximate formula Fe S and that there may also be obtained mixtures of this material with iron or with oxide of' iron, or mixtures of FeS with iron or oxide of iron, by the inclusion of an excess of hematite.

' With the proportionate amount'ofreaction coke and surplus iron in a shaft furnace I may makea material whichvcomprises a percentage of free iron, apparently in solution in ferrous sul hide. Thereis a possibility that eutectic mixtures are frequently formed; and I'have noted that the ferrous sulphide formed under advantageous conditions in the shaft'furnace has'a melting point lower than ferrous sulphide in a pure state has. This may indicate the presence in solution of oxides of iron and possibly other substances, while at times theremay be formed eutectics of oxidesand/oriron, andsulphide of iron. In any event I have found that my heat treating process works with all of the various ferrous sulphide materials, and I have also found that it works very well with mixtures of. ferrous sulphide and other materials. Myexperiments have shown that it is per- ,fectly successful in producing a solid casting comprising 50% ferrous sulphide and 50% hematite, which have been meltedup together, I

While I do notwish to be bound by my present theories respecting the'behavior of ferrous sulphide materials in my heat treating process, I explain this result upon the theory that my heat treatment is effective with ferrous sulphide materials as such and that the soundness of a casting in which I have eliminated disruptive internal strains is not affected by the mere presence in intercrystalline spaces of extraneous substances such, for example as ferric oxide. I have already pointed out that these ferrous sulphide materials have relatively smooth expansion curves and cooling curves; and so 'far as I amaware, there are not points at which during cooling sharp crystalline changes occur.

The problem is essentially one of the neutralization of the internal strains which result from cooling so that they may be reduced below the tensile strength of the material, and of the closing up of the grain structure as shown in the photonncrographs so as to produce a strength.

I accomplish'the object of my invention by that series of steps which'I shall now more fully'se t forth and claim. I

I prefer to take the moltenferrous sulphide 'material from the cu'pola or shaft furnace or other melting device {in the ordinary ladle and to let it remain inthe'ladle or other con- .tainer until it has cooled to'fa temperature product of much greater tensile close to it's'freezin point. As it comes from the shaft furnacet e material will ordinarily be somewhatsuperheated. As it cools, it will change from white gradually to yellow, and

from yellow gradually to orange. I prefer to cast in thedeeper orange phase, if solidification isto be allowed in a mould in the open a1r. I have found that I may, however, produce sound castings at a pouring temperature ranging from considerable superheat to a point-where some crystals have begun to form in the ladle while the main body of the ferrous sulphide materialis sti'll'fluid. If crystals have commenced to form, of course, solidification'is more rapid, the casting must be done rapidly'and in ordinary commercial operations it is not possible to control the time in relation to the temperature so closely. Therefore, the material is ordinarily cast at a temperature somewhat higher than that at which crystals begin to form.

The molten material is poured into moulds which for my purpose are preferably of cast iron or steel, because of the moderate degree of chilling obtainrhalthough moulds of refractory materials may be used also. They may be in the shape of open pans or they may be in the shape of open top moulds in which the cooling rate, is permissible.

and hence forms the casting itself with a maximum of initial tensile strength.

When the casting in the mold is frozen or solidified, 1' permit it to cool somewhat further, but I do not carry my cooling to the heat treating furnace where the temperature is kept constant at 1100 F. for, roughly, two hours. moved to another section of the furnace in which the temperature gradually falls, final-'- ly reaching room temperature in from one to one and one-halfhours. Some variatio'n .in

The preferred 'embod1ment and heat treat'mgiprocess comprises casting at a relatively low temperature and freezing moderately rapidly in the mould, for the purpose of producing a fine grained crysta line structure. In this cooling process the nucleus of some strains will be set up because of the variation in temperature from center to edge of the material, after it solidifies. Before these strains have increased beyond the materials tensile strength, and before the casting has cooled very far I treat it to bring about an equalization of temperature in all its parts. The temperature to which I heat it is such that all strains,including the accompanying temperature equalization strains, will not rupture thematerial but will themselves be neutralized.

The selection of this temperature is vital. It must be high enough, of course, to bring about such a-condition in the material as to permit the neutralization of strains. I believe this to be a point at which the material is slightly plastic. But in addition to this it must not be such as'will independently set up strains in the material greater than its tensile strength'will stand. A variation beyond certain limits from the optimum temperature, as hereinafter explained, will not produce good results. If the temperature is too low the strains will not be neutralized; if too high, new strains develop upon coolmg.

I believe that at the temperature of 1100 F. or thereabouts the ferrous sulphide material is veryslightly plastic,ri. c. has enough.

flow to permit these strains to adjust themselves, and I find that such adjustment-takes place, roughly, in one and one-half tortwo hours of heat treatment. During this heat treatment all parts of the casting are brought to the same temperature, and if I thereafter cool carefully sothat the temperature fall takes place without excessive variation from center to edge, I find that I produce absolutely sound anodes of any convenient size Following this, the casting is re-- for use, such as anodes weighing 100 pounds apiece. Y I

I have made sound castings by heat treatment at 107 5 F. but the best results are obtained at 1100 F. The tendency is,- if heat treatment is attempted much below 1100 F.,

for the casting to rupture while in the furnace. If, however, temperatures much above '1100 F. are used for heat treatment, the casting does not crack in the furnace butdevelops numerous edge cracks on even extremely slowcooling such as in twelve hours. Thesetemperature measurements were made with standard thermocouple apparatus and are of course subject tothe usual errors, if any, inapparatus of that type.

It is desirable that the pouring temperature should not be too much above the freezing of my .casting' point because in that case the rate of solidification is slow, and a course, weak crystalline structure is obtained. Pouring at a higher temperature with a subsequent chilling in the mould freezes the material quickly, but at the same time sets up so much temperature variation in the casting that the material breaks up before or whilethe heat treating. is being done.

There is, therefore, a maximum amount of superheat that can be tolerated and a more or.

less definite rate of solidfication which is desirable. In my experience this rate is obtamed by open air solidification in a mould ,having walls, roughly, one-half inch thick.

If the cast piece is placed in the heat treating furnace too late, so that the temperature of the interior of the casting is much below 1370 F., it breaks up after being placed in the heat treating furnace. Of course, when the body of the casting is at 1370 F., the edges will be somewhat cooler. p 1

The reason for the formation of sudden cracks upon simple cooling seems to be that the outside of the casting, cooling first, contracts while the inside, being'warmer, tends to maintain its original size, so that the outer shell is split apart. The reason for the cracking of a casting in the heat treating furnace may be that as the piece is heated up, the temperature equalizatlon sets up strains in the casting some parts of which had cooled below the slightly plastic stage. In any event the casting must be placed inthe heat treating furnace before it has cooled too far'. By

careful dilatometer tests, I have found the coeflicient of expansion of the iron sulphide material to be, roughly, three timesthat of iron, which fact, when considered in connection with its relatively very low tensile strength indicates the magnitude of theproblem solved by my heat treating process in the production of sound castings.

While the casting is in the heat treating furnace at 1100 F., the whole is brought to a uniform temperature and all strains are apparently released. The material, I believe,

closer grain, denser and much tougher.

After the heat treatingprocess it mustnot be chilled too quickly or the unequal tempera ture in various parts will set upenough new strains to rupture the material.

If an unl1eat'-treated, cold sample 1S slowly brought up to. 1100 F., and then allowed to cool slowly, numerous cracks develop on cooling from the edges toward the center. This is because the material is not strong enough to stand the strains set up by deformation due to differences in temperature between center and outside as the matcrial cools. My heat treating process, however, puts the material into such condition and makes it so strong that itcan be cooled down fairly rapidly without any cracking, as I have before indicated.

I have found that by removing the mould and the casting together from the'furnace to the open air in moderate Weather and in the absence of drafts, with the open top covered by a steel plate, a sound casting can be produced; but for commercial operation where it is not advisable to take any chances of the .ii cracking of anodes, it is advisable, as I have already indicated, to place the casting in a portion of the furnace where the temperature gradually falls from 1100 F. to room temperature in an hour or possibly more.

After my heat treatment, I have found that the average strength of the ferrous sulphide 9 crease of 50%.

5 cro hoto ra hic material (and by this I mean material consisting almost wholly of ferrous sulphide of iron) is increased from 450 pounds per square inch to 675 pounds per square inch, an in- Itresists breaking with a hammer much better, and is strong enough to be handled in large, thin slabs. The heat treated material appears closer grained, denser and tougher, as will be seen in the mirepresentations forming Figures 1 and 2 of the drawing. It is possible that other changes take place in the material, which I have not yet determined by experiment, such possibly, as an actual in- 9 crease in specific gravity and a contraction in volume.

Heat treating furnaces for the accomplishment of'my purpose are within the capabilities of those skilled in the art to construct. They ma be of various forms, as will be clear, an

ried in different types of heat treating furnaces without departing from the scope of. f -my invention. U

Having thus described my invention, what I claim as new and desire to secure by Letters Patent, is

1. A process for obtaining sound castings of a material comprising ferrous sulphide,

5 which comprises casting said material and my process may be somewhat vapermitting solidification thereof in a mould and thereafter, without permitting said solidified casting to cool until internal strains set up by said solidification and cooling become greater than the tensile strength of said cast} ing so as to cause cracks therein, subjecting the said casting to a degree of temperature short of the liquefaction point but suflicient to render it slightly plastic, for a sufficient time to permit the neutralization of internal strains and the closing up of grain structure, and then coolirg said casting slowly and evenly to room temperature.

2. A process for obtaining sound castings of a material comprising ferrous sulphide, which comprises casting said material at a temperature. close to its freezing point and permitting the quick solidification thereof in a mould so as to obtain a fine grain of crystallization in the casting, and thereafter, with out permitting said casting to cool until internal strains set up by said solidificationand cooling become greater than V the tensile strength of said casting so as to produce cracks therein, subjecting said casting to a degree of heat short of the liquefaction point thereof but sufficient to render it slightly plastic for a period of time sufficient to permit the neutralization of internal strains and the improvement in grain structure resound castings ferrous sulphide, said material and allowing it to cool rapidly while freezing to a temperature above 1300 F., at the center, then transferring the casting to a heating zone and there maintaining it at a constant temperature higher than 1000 F. but below its liquefaction point for about two hours, then cooling said casting evenly to room temperature in about one hour. r v

4.. A process for obtaining sound castings in a material comprising ferrous sulphide, which comprises cooling said material before casting until it becomes a ing said material while still molten, and allowing it to cool rapidly in the moulds While freezing to a temperature at which the body of said material is at a bright red heat and the edges tlieicofbegin to darken, then placing said casting in a heat treating zone and there maintaining it at a constant temperature higherthan 1000 F. but below its liqueaction point for about one and one-half hours, then cooling said casting evenly to room temperature in not less than one hour. I

deep orange, castmoulds in the open air and permitting it to cool therein until solidified and until it reaches a temperature around 137 0 F., thereupon transferring the casting to a heat treating furnace and maintaining it there for not less than one and one-half hours at a temperature of 1100 F., then cooling said casting evenly to room temperature in not less than one hour. a V

6. A sound casting of a material consistingessentially of ferrous sulphide, said casting having as the result of a. heat treatment atensile strength at least 675 pounds per square inch.

SAMPSON 'W. YATES. 

